That's setting the virtual IP address of the local VPN, but doesn't connect to a remote VPN. You can however, as you know, use a SOCKS5 proxy.

OK, thx for reply. So I'm totally wrong :( But than I don't know the benefit of this virtual IP address of the local VPN ?

Sometimes, you local internet router might use the same IP address as TC, then you run into conflicts and need to change the IP of TC's local VPN.

OK but what if I use a VPN like it is intended? I.e. to have access to other devices on the VPN.

I don't understand what you are meaning. So I hope my answer will fit:
Android is allowing only 1 VPN at a time. As soon as you activate an other VPN, TC will stop working

I'm also interested in this. I need a VPN to connect to my house when I'm out, but in doing so I'd have to disable TC, which I don't want to do. Currently I've got a small number of my apps being port forwarded from ssh in termux, and I've been looking at using an android for work profile, but it would be very nice to have first-class support for sending traffic through another app, or through a built-in proxy.

you can install you VPN app and all apps you need to route thru VP in shelter Work profile. That profile is isolated from main profile apps which will be monitored by TC

and here is another info from TC telegram group (which is better for support)

This is another explanation that used Shelter or Island: https://itsignacioportal.github.io/netguard-pdnsf-any-vpn-combo/

The guide additionally sets up personalDnsFilter, which I think is optional if combined with TC. It allows, however, to use a regular VPN with TC.